The unmanned craft malfunctions and veers off course from its Mars moon destination. It could be 'most toxic falling satellite ever,' one expert says.

Reporting from Moscow — Russian controllers battled Wednesday to redirect a space probe stuck in a low orbit, raising fear that it could crash back to Earth.

The $167-million unmanned Phobos-Ground spacecraft was launched early Wednesday from Baikonur cosmodrome in neighboring Kazakhstan. But when the probe separated from its booster rocket, the engines did not fire to put it on the path to Phobos, one of Mars' two moons.

"We had a hard night because for a long time we couldn't detect the spaceship," Vladimir Popovkin, who heads the Roskosmos space agency, told reporters. "It was established that the propulsion engine failed to work."

He said he hoped that the malfunction was the result of a programming error rather than equipment failure, which "could in no way be fixed from Earth."

It was the latest in a series of failures for Russia's space program. The problems also have the potential to affect U.S. space efforts.

Since the end of the U.S. space shuttle program in July, NASA has depended on Russia to transport U.S. astronauts to and from the International Space Station. The U.S. hopes to regain human space flight capability with a privately developed launch system to service the ISS. That system is scheduled to start operating by the middle of the decade.

In December, three Russian navigation satellites failed to reach orbit because they were loaded with excess fuel and tumbled into the ocean. In April, the Russian space agency lost contact with a military satellite. And in August, the Russian Progress cargo spacecraft crashed after an abortive launch to take supplies to the International Space Station.

Space agency officials said they had about two weeks to redirect Phobos-Ground before its power sources run out and a relative narrow window of time to reach Mars closes. If they succeed, it should reach Mars' orbit in 2012 and will collect ground samples from Phobos to bring back to Earth. It is Russia's first interplanetary mission since the Mars 96 probe crashed shortly after launch in 1996.

If they fail, experts worry that tons of toxic fuel carried by Phobos-Ground could turn it into one of the most dangerous spacecraft to fall from orbit.

"About 7 tons of nitrogen tetroxide and hydrazine, which could freeze before ultimately entering, will make it the most toxic falling satellite ever," James Oberg, a NASA veteran who now works as a space consultant, said in an email to the Associated Press. "What was billed as the heaviest interplanetary probe ever may become one of the heaviest space derelicts to ever fall back to Earth out of control."

But Oberg told the AP that it was still possible to regain control of the probe, saying, "Nothing irreversibly bad has happened."